Monoclonal antibodies were used to examine the expression of a number of carbohydrate antigens in the rat endometrial epithelium from day 1 to day 8 of pregnancy and in ovariectomized rats supplemented with ovarian steroids. Carbohydrate antigens based on the Galβ1–GlcNAc backbone structure were expressed and some of these (Ley, Lex, B antigen) were present at all stages of pregnancy and independent of ovarian steroids. The H-type-2 antigen was stimulated by progesterone and expressed in the sensitized and receptive uterus, but was not detected after implantation or, in ovariectomized rats, in the refractory phase. The H-type-1 antigen, which is stimulated by oestrogen in the mouse, appeared to be stimulated by progesterone in rats. It was expressed throughout the period of pregnancy but maximal expression was found on days 4–5. The histo-blood group A antigen appeared in ovariectomized rats only after treatment with progesterone followed by three daily injections of progesterone and oestrogen, and in the corresponding postimplantation period of pregnant rats. Its appearance corresponded to the loss of detectable H-type-2 antigen. This study shows that the rat endometrial epithelium expresses some carbohydrate antigens not expressed in mice (A and antigen) or under completely different steroidal regulation (H-type-1). Moreover, the antigen was expressed on the endometrial epithelium adjacent to decidua on days 7 and 8 of pregnancy, but not in rats given ovarian steroids to mimic the sensitized, receptive or refractory phase. Differences in expression between glandular and luminal epithelia indicated differences in steroidal regulation, as found in mice.